Microprocessor technology continues to evolve competitively, as widely reported in the press. Criticism has been voiced in the trade press, however, about the inability of mass storage devices such as disk drives, CD-ROMs, and DVD drives, to name a few, to keep up with the advancing speed of the microprocessors. Mass storage is viewed as lagging the advances achieved with respect to microprocessors. Particularly, advances in the operational speed and storage capacity of mass storage devices would be welcome.
Electron emitters that create electron beams present the basis for alternative possible solutions for memories, electronic chip fabrication equipment, and other devices. So-called cold cathode electron emitters have attracted the attention of many manufacturers. Several problems exist in converting this cold cathode technology to products. One such problem is the creation of an electron focusing structure that can be used in multiple applications that require a high density of emitting devices such as with mass storage devices.
A typical tip or flat emitter driven memory device is based upon the controlled use of electron emissions from an emitter. An emitter emits electrons in response to an electrical signal. Focused emissions can affect various media to produce, for example, memory and lithography effects. These and other applications require the use of controlled and focused electron beams. Production of such beams involves the fabrication of an emitter and focusing structure, typically an electrostatic lens.
Various emitter driven devices make use of a target anode medium. The target anode medium is the focus point for the controlled emissions. A target anode medium is held at hundreds of volts differential from the emitter/cathode structure. Alignment and focusing length are important issues in emitter driven devices. Fabrication of lenses on emitter chips requires the precise alignment of the emitters and focusing elements. To achieve alignment, standard practice for micro-fabricated emitters is to form the entire lens and emitter structure in a single self-aligned photostep. This achieves good lens/emitter alignment, but limits the distance of the lens from the emitter and also limits the thickness of the lens. Generally, the lens is between one and three times the distance from an extractor as the extractor is from a tip emitter formed in a well. The focusing length is accordingly limited to the short distance afforded by the separation of various metal layers in an emitter/focusing lens chip.
The single self-aligned photostep process further sets the diameter of the lens to that of the well; since both are formed from the same etch. Due to the common size of the extractor and lens and their relative positions, the divergence angle of the emission beam from the emitter is wider than the lens. Small lens diameter additionally generates large spherical aberration and limits minimal spot size. This adversely affects the ability to produce tightly focused spots from the emissions. A tightly focused spot size, e.g., less than 35 nm, is desirable to increase density of a memory and a narrowly diverging beam is desirable for a scientific instrument or a lithography tool. Some laboratory memory devices have achieved bit sizes of about 50 nm and conventional flash memories have bit sizes exceeding 100 nm.
The conventional well housing a tip emitter also is deep. The standard small lens size requires an extremely precise alignment, ˜0.04 μm, between the tip/extractor and lens. As mentioned, a single etch produces the well and lens to achieve the alignment, and aspect ratios of the well are generally high, about 2 to 1 (depth to diameter). This creates processing complexity because it is harder to deposit a tip emitter in a deep well as the coating of the inside of the well is to be avoided while forming the emitter tip. This may require more sophisticated tooling, e.g., larger evaporators. Deep wells can also produce poor emitter tip quality and low yields. There remains a need for an improved electronic device that makes use of focused emissions from emitters.